The present invention relates to a system and method of manufacturing the same, for efficiently purifying indoor air by combining forced (variable speed) air movement (fan and motor), filtration, ultraviolet germicidal irradiation, and photocatalysis in an air purifier unit.
Some of the earliest published references to titania (titanium dioxide or TiO2) photocatalysts are by Formenti, M., et al., “Heterogeneous Photocatalysis for Oxidation of Paraffins”, Chemical Technology 1, 680-686, 1971 and U.S. Pat. No. 3,781,194 issued Dec. 25, 1973. Since the 1972 discovery of the photocatalytic splitting of water on titanium dioxide electrodes, by Fujishima and Honda, the science and technology related to heterogeneous photocatalysis in both water and air has been extensively studied and is the subject of numerous patents and scientific publications. Both the physics and chemistry of heterogeneous photocatalysis remain areas of active investigation. Much of the early work of relevance to this patent is summarized, by Kittrell in U.S. Pat. Nos. 6,179,972 and 6,221,259, as well as, Peill, et al., in U.S. Pat. Nos. 5,875,384 and 6,051,194. Despite investigation of many alternatives, the anatase crystal morphology of titanium dioxide remains the photocatalytically active semi-conductor of economic choice, although many claims of additive enhancements have been and continue to be made.
Considerable historical effort has been expended to maximize photocatalytic activity of anatase TiO2 by minimizing particle size (to maximize effective surface area) while maintaining strong adhesion to substrate surfaces. It is well known and documented that photocatalytic activity is directly related to (a) the intensity and wavelength of irradiation at the illuminated photocatalyst surface area, (b) the magnitude of the illuminated photocatalyst surface area, (c) the rate of flow of contaminants across the illuminated photocatalyst surface area (irradiated surface contact), and (d) in air, the “absolute” humidity of the ambient air. The “quantum or photocatalytic efficiency” relates to the fraction of light-source photons that are effective in causing the photocatalyzed reactions.
Considerable effort is currently being expended, in the field of photocatalysis, to enhance the photocatalytic efficiency of anatase titanium dioxide with various catalytic additives (as described in many of the patents cited, e.g., U.S. Pat. Nos. 6,409,928, 6,179,972, and 6,221,259) and to extend the wavelength of photocatalyst-activating irradiation into the visible wavelength range, as described in U.S. Pat. Nos. 7,153,808 and 7,175,911.
The objective of the present invention is to provide a large illuminated photocatalyst surface area while ensuring intimate air-surface contact, at all air flow rates, so as to maximize the photocatalytic efficiency of the light source. Where the photocatalyst substrate fibers are high-purity quartz, wool, mat, or felt, the fibers act as elementary waveguides having a UV-conductive core with a reactive outer coated surface. Refraction and reflection (both internal and external) of randomly oriented fibers ensure efficient distribution of light photons throughout the fiber mass, until absorption at the photocatalytic coating occurs.
Early work with photocatalyst powder coatings encountered particle size minimization and bond-to-substrate issues. Various high temperature coating application techniques have been technically successful but are economically and practically prohibitive for coating fiber wool, mat, or felt. Organic binders for powders, such as methylmethacrylate and various organic resins, have been found to emit unacceptable odors under UV-C irradiation. Titanium oxide films formed by inorganic peroxotitanium hydrate sol gels together with a peroxotitanic acid binder have been found to have good substrate-bonding and photocatalytic properties, while remaining odor-free under UV-C irradiation.
Photocatalyzed reactions of volatile compounds (VCs) are known to be strongly endothermic, such that the photocatalyst-activating energy output of commonly available lamps limits the concentrations of treatable airborne VCs to parts-per-million (ppm) or less. The concentrations of most noxious/offensive odors and airborne pathogens are within this treatable range.
U.S. Pat. No. 5,330,722 discloses, in one embodiment, a longitudinal arrangement of a disposable, hollow, cylindrical air filter rotated about the longitudinal axis between two closely spaced UV lamps (one irradiating the inside wall and one irradiating the outside wall. Electrostatic but no photocatalytic enhancement is claimed. Air flow, as in the present invention, is from the outside to the interior (core cavity) of the filter cylinder. The germicidal filter medium is claimed to include fibrous materials (including fiberglass) and pleated paper suitable for use in filtering air. The ultraviolet radiation sources claimed include ozone producing lamps. There is no discussion of UV-induced deterioration of filter materials.
U.S. Pat. No. 5,616,172 discloses a longitudinal but rectangular arrangement of UV lamps within the enclosed housing of an air treatment system. There is neither photocatalytic activity nor disposable cartridge claimed.
U.S. Pat. Nos. 5,779,912 and 6,409,928 disclose a photocatalytic method and apparatus for mineralizing organic contaminants in water or air. The photocatalytic performance is claimed to be enhanced by addition of an oxidant stream to the contaminated fluid stream as well as addition of metal catalysts to the photocatalyst coatings. With some similarities to the present invention in concept, the differences are: (a) the focus of these patents upon contaminated water, (b) the photocatalyst coating is applied only to the directly irradiated surface, and (c) a supplementary oxidant (oxygen or ozone) stream is directed to the reaction zone. Many reasons are stated for rejecting an annular photo-reactor concept, such as the present invention.
U.S. Pat. No. 5,833,740 discloses a longitudinal, coaxial combination of UV-V and UV-C light sources within the enclosed housing of a chemical and biological air purifier. There is neither photocatalytic activity nor a disposable cartridge claimed.
U.S. Pat. Nos. 5,875,384 and 6,051,194 disclose the construction and performance characteristics of oriented TiO2-coated fiber optic cable reactors (FOCRs) and their use in the degradation of organic and inorganic pollutants in both air and water. The fibers described were 400 to 1,000 microns in diameter and were maintained straight and parallel within the reactor. Photocatalyst particles were baked onto the fiber surfaces after dipping into a TiO2 slurry. Due to weak photocatalyst-fiber bonding, there was a concern for de-lamination of the coating on fiber-to-fiber contact. Therefore, spacers were utilized to prevent fiber-to-fiber contact. The UV light source was unspecified.
U.S. Pat. No. 5,997,619 discloses an annular filter arrangement preceding UV germicidal irradiation. No photocatalysis is claimed.
U.S. Pat. No. 6,053,968 discloses an annular portable room air purifier in which the germicidal UV lamps are arranged at the outer periphery of the core cavity, not on the longitudinal axis. HEPA filtration but not photocatalysis is claimed.
U.S. Pat. Nos. 6,179,972 and 6,221,259 disclose a two-stage process, catalyst, and apparatus for photocatalytic conversion of contaminants in oxygen-containing air and water streams combining a UV-activated photocatalysis stage and a subsequent high temperature catalysis stage. The photocatalyst is described as including various combinations of titanium and zirconium, both supported by silica “gel”.
U.S. Pat. Nos. 6,235,401, 6,344,277, 6,344,278, and 6,379,811 disclose the method of preparation of a yellow transparent jelly (viscous) amorphous type titanium peroxide sol which serves as an excellent binder for titanium dioxide powders and sol gels. These patents further teach that when the titanium peroxide sol is heated at 100 degrees C. or more for several hours, the anatase type of titanium oxide sol is obtained. When a substrate is coated with the amorphous type titanium peroxide sol and then dried and heated at 250 to 940 degrees Celsius, an anatase type of titanium dioxide is obtained. This material is similar to the preferred photocatalyst material of the present invention (without the necessity of heating).
U.S. Pat. No. 6,309,611 discloses a multi-stage photocatalytic reactor for both gaseous and aqueous streams, each stage of which is somewhat similar in concept to the present invention with the following differences:                1. The air flow through the photocatalytic medium is reversed (out from the illuminated side as opposed to into the illuminated side).        2. There is no described pre-filtration.        3. The preferred photocatalytic medium substrate is a cellulosic-fiber fabric material (cotton or any bio-polymeric) to which the titania particles are claimed to be hydrogen bonded in a flexible stocking or a rigid metallic or ceramic screen.        
U.S. Pat. No. 6,309,611 provides an excellent background discussion of prior relevant art, patents, UV light sources, mass transfer considerations, photocatalytic media, and provides an internal classification (by type) of catalytic media.
U.S. Pat. No. 6,358,374 discloses an elaborately schematic integrated photocatalytic gas purifier and an adsorbent bed (gaseous contaminant accumulator). The contaminants are first adsorbed (accumulated/concentrated) in the bed until saturated. The adsorbent bed is then regenerated by heating to release the captured contaminants which are released into a fixed volume of gas that is re-circulated through the heated bed and through a photocatalytic gas purifier which oxidizes the released contaminants. Unfortunately, the patent discloses no details of the photocatalytic gas purifier, light source, or photocatalyst.
WO/2002/004036 discloses a method and apparatus for air purification that consists of a cylindrical arrangement of an inner number of UV light sources (illumination section), preceded by pre-filters (pre-sterilization section). An antimicrobial agent may be coated on surfaces in one or both the filter and illumination sections. Design-induced turbulence is claimed to increase the dwell time of contaminants in the illumination section. However, dwell time is determined by flow rates, not turbulence. Turbulence does increase the possibility of surface contact. No photocatalysis is claimed.
U.S. Pat. No. 6,358,374 discloses a method for preparing a thin film of oriented anatase crystals on a substrate by spraying a vaporized titanium alkoxide (in an inert carrier gas) onto the heated substrate surface, at atmospheric pressure. This method and process is not used in the present invention.
U.S. Pat. Nos. 6,228,502 and 6,465,042 disclose a chemical vapor deposition technique for producing an oriented crystalline film of anatase titanium dioxide on a substrate surface, followed by annealing in an oxygen atmosphere, and then coating with silver, or copper, or oxide thereof.
WO/2002/083307 discloses an annular photocatalytic air purifier comprising: (a) a tubular housing having an inner and an outer wall, a central axis, a first end having a centrally located air intake nozzle, a second end having at least one air exhaust port; (b) an air exhaust plenum between the inner housing wall and a radial porosity medium, the porosity medium extending radially and axially about the axis; and (c) a housing central portion defined by an interior perimeter of the radial porosity medium, the central housing enclosing an ultraviolet lamp and a packing medium, the packing medium extending radially and axially about the lamp and comprising a plurality of spiral wound filaments (semi-transparent to UV light, E-glass, a form of fiberglass, and of 15-25 microns diameter) coated with a photocatalytic film. The substrate/support fibers are claimed to be any of glass, metal, plastic, nylon, or other material that can be assembled into small fibers. Such fibers are claimed to be organized into a twisted strand “bottle brush” configuration, bound by at least one stainless steel wire. Although geometrically similar to the present invention, the major differences are:                1. No pre-filtration is claimed, in contrast with the present invention.        2. The fiber orientation is regular (“bottle brush”), not random (wool, felt, mat), as in the present invention.        3. The fibers are, at best, semi-transparent to long wavelength UV light and opaque to short wavelength UV (germicidal) and not fully transparent, as in the present invention.        4. The direction of air flow is inside-out, not outside-in.        
WO/2002/102497 discloses a photocatalytic air purifier apparatus that consists of an annular arrangement of an inner photocatalyst-coated transparent sleeve (“member”) enclosing the axial UV light source with both enclosed by either a photocatalytic cylindrical surface or “a plurality of tubes”. Air flow is directed through the annulus between the transparent sleeve and the outer photocatalytic cylinder. Photocatalytic activity is limited by the smaller UV-illuminated photocatalyst-coated surface area and lesser intimate contact between airborne molecules and the smaller photocatalyst surface than in the present invention.
U.S. Pat. No. 6,531,100 discloses various metal wire mesh, cloth, and non-woven substrates, surface enhanced with particles of many materials bound to the substrate surface, on coating with photocatalyst, to maximize the area presented to a UV light source. Baking or sintering, after wet application of photocatalyst sol, is said to provide a secure photocatalyst bond to the substrate material. The patent also describes various arrangements of planar photocatalyst-supporting bodies and UV light sources in photocatalytic apparatus.
U.S. Pat. No. 6,589,489 discloses an air purifier of similar geometry to the present invention but incorporating a “dielectric body” in place of the photocatalytic cartridge of claim 1 and utilizes both UV1 (UV-V) and UV2 (UV-C) light. In this invention, the dielectric body may be made of, for example, quartz, or alumina fibers or silica granules or sponge so that it is porous to air and transmissive to UV light. The dielectric body of this invention may contain photocatalytic material and electron transfer is claimed to be enhanced by and electric field imposed by concentric anode and cathode metal mesh cylinders incorporated within the dielectric body. Air flow is from the inside→out (of the dielectric body) in contrast with the flow from the outside→in (of the photocatalytic cartridge) in the present invention.
U.S. Pat. No. 6,602,918 discloses processes for producing titanium oxide coating agents for the purpose of forming a titanium oxide film on a substrate with improved adhesion and increased density. The product of this invention is very similar to that used in the present invention.
U.S. Pat. No. 6,730,265 discloses an air UV disinfection device and method involving a UV light source connected by fiber optics to a gas purification zone (reaction chamber of gas purifier) that may involve reflective or photocatalyst-coated interior surfaces. This patent also claims “particle arresters” (fiber filters) composed of fibers selected from the group consisting of glass fibers, acrylic fibers, quartz fibers, paper fibers, cellulose fibers, cotton fibers, plastic fibers, and cominations thereof. Gas flow and control of that flow through the gas purification zone is not detailed.
U.S. Pat. No. 6,602,918 discloses a small (suitable for plugging directly into an electrical outlet) apparatus and method for purifying air consisting of a chimney, UV light source, a doped (platinum metal, etc.) photocatalyst (primarily titanium dioxide) coated on internal walls or fibrous mass (unspecified). Air flow through the unit is maintained by convection created by internal heating. Convection alone is a slow process that generates a small number of air exchanges per hour in even modest-sized indoor environments.
U.S. Pat. No. 6,764,655 discloses a light-leakage type photocatalytic filter comprised of longitudinally bundled photocatalyst fibers with inter-fiber gaps to permit fluid communication pathways. Light introduced at the ends of the fibers travels along the photocatalyst fibers while partially leaking therefrom and thus causing photocatalytic reactions. According to the patent, fluid flow may be parallel, perpendicular, or inclined with respect to the longitudinal direction of the fiber bundle. A bundle consists of about 10,000 fibers of 125 nm in diameter and 200 mm in length. Neither the photocatalyst nor the composition of the fibers is disclosed.
U.S. Pat. No. 6,773,683 discloses a schematic concept for photocatalytically oxidizing sulfur dioxide to sulfur trioxide using a UV laser or lamp light source connected, via fiber bundles, to a reactor but without disclosure of photocatalyst or details of reactor design.
U.S. Pat. No. 6,783,578 discloses an air purification apparatus that includes a HEPA or ULPA cartridge filter and UV lamp configuration, geometrically similar to the present invention but without a photocatalytic medium and with the air flow reversed (from the inside of the cartridge to the outside).
U.S. Pat. No. 6,835,679 discloses a method and apparatus for light curing of composite materials incorporating one or more “lossy” optical fibers. This patent teaches that optical fibers may be made lossy by bending the fiber, weaving the fiber into a mat (to create periodic micro bends), or by removing the fiber cladding. Such light losses (leakage) distribute the light throughout the composite material.
U.S. Pat. No. 6,884,399 discloses a modular photocatalytic air purifier intended for installation in heating, ventilating, and air conditioning systems (fan coil units). UV lamps are positioned between photocatalyst-coated filter elements such that the filter elements serve as the photocatalytic surfaces. The filter geometry is stated to include honey-combs, fins, mesh, a filter-type structure, a fibrous type, or a filamentous structure.
U.S. Pat. No. 6,884,752 discloses an aqueous composition of titanium dioxide that includes varying amounts of acrylic aliphatic urethane polymer to improve wetability of the substrate, as well as, thinness and clarity of the produced film.
WO/2005/039659 discloses a germicidal air treatment method and device for air purification that consists of a longitudinal arrangement of UV light sources, inside an impermeable-walled enclosure, preceded by serial pre-filters (pre-filter, HEPA filter, and possibly a carbon filter), and equipped with a fan and motor. An ionizer is located downstream of the fan. UV irradiation is claimed for at least one side of one filter. The UV irradiation field within the UV treatment chamber is maximized by highly reflective walls (rough-surfaced aluminum, in the preferred embodiment). Components are arranged for turbulent air flow. This invention relies on filtration in place of photocatalysis, unlike the present invention.
U.S. Pat. No. 6,902,653 discloses fluid-permeable reticulated or packed bed photocatalytic reactors in which both the substrate and semiconductor photocatalyst are semi-transparent to the activating light. Fluid flow is either parallel or perpendicular to the lamp axis. The light sources are embedded within the packed bed.
U.S. Pat. No. 6,939,397 discloses a UV-based air-purifying apparatus of similar geometry to the present invention but involving an impermeable-walled, non-filtering, reflective-surfaced cartridge without any photocatalysis.
WO/2006/018949 discloses an air purifier and method for air purification that consists of a tubular (circular or polygonal section) main body having a plurality of air movement holes on the wall surrounding the inner space and an internal UV light source illuminating the inner wall surface. The formed, porous main body may be made of any photocatalyst-supporting fiber material, including cellulose and acrylic fibers. By forming projections and recesses on the inner surface, the irradiated surface area is increased. Air movement (in and out of the air movement holes), in the preferred embodiment, is provided (slowly) by convection resulting from lamp heat and diffusion. However, some forced air movement is suggested to be provided near the air movement holes by a discharge or intake fan. No filtration is claimed.
U.S. Pat. No. 7,063,820 discloses a method for disinfecting fluids on photocatalytic surfaces excited by UV-A irradiation within heating, ventilating, and air conditioning systems or in air ducts. An electrical bias applied to the photocatalytic elements is claimed to enhance the photocatalytic activity at low ambient humidity.
U.S. Pat. No. 7,083,659 discloses a combined light source and non-photocatalytic air purifier unit that has a cylindrical filter surrounding the fan. Therefore, the air flow through the filter is the reverse of the present invention and no photocatalysis is involved.
U.S. Pat. No. 7,144,840 discloses a method of preparation of small titanium dioxide crystals in a sol using polyethylene glycol as a dispersing agent before coating substrates and calcining the coated substrate.
U.S. Pat. No. 7,175,814 discloses an annular air disinfecting system and cartridge device containing ultraviolet light. While of similar annular geometry to the present invention, no photocatalysis is contemplated in this patent (only reflective surfaces). In this patent (unlike the present invention), the light source and other electronics are incorporated into the cartridge unit.
U.S. Pat. App. No. 2007084350 discloses a self-contained photocatalytic air purification apparatus comprised of a outer housing with air inlet and outlet ports enclosing sets of UV light sources illuminating both sides of one or more internal filters. Internal surfaces may be polished metal or coated with photocatalyst. Air flow is directed around a series of air baffles “to cause turbulence and to maximize exposure time.” Exposure times are determined by air flow rates, not turbulence. Nevertheless, turbulence does improve the possibility of surface contact. In this invention, photocatalytic activity is limited by the smaller UV-illuminated photocatalyst-coated surface area and lesser intimate contact between airborne molecules and the smaller photocatalyst surface than in the present invention.
U.S. Pat. App. No. 20060086252 discloses a ceiling-mounted, single-sided, planar arrangement of UV lamps, photocatalytic filter, and active carbon coated filter in an air purifier. No detail is provided regarding the lamps or photocatalyst.
U.S. Pat. App. No. 20060086252 describes a photocatalytic air purifier comprised of a conventional lamp and shade arrangement in which the inner surface of the lamp shade is coated on the inner illuminated surface with a photocatalyst (broadly specified) such that heat generated by the light source (broadly specified) causes convection of air past the lamp shade surface. Convection alone is a slow process that generates a small number of air exchanges per hour in even modest sized indoor environments.
As noted above, organic binders for photocatalytic support structures are unsuitable as they emit unacceptable odors under UV-C irradiation. Further, Applicant has noted a heightened manufacturing cost associated with cylindrical support structures. Therefore there is a need for an improved and economical photocatalytic structure and apparatus implementing same.